The present invention relates in general to apparatus for detecting radioactivity and more specifically to a gamma camera having equalized sensitivity across detected images.
As a medical imaging modality, gamma cameras are employed for measurement of gamma rays emitted by radioactive pharmaceuticals concentrated in a body. The camera creates an image based on the distribution of radioactive emissions.
A particular type of gamma camera which employs a scintillation crystal to convert gamma events into visible photons and an array of photomultiplier tubes to detect the photons is known as the Anger camera. This device determines the position of each interaction of a gamma ray with the scintillator by measuring the relative visible light energy transmitted to each photomultiplier tube. The sum of energies from all photomultiplier tubes corresponding to an event gives a representation of the energy of the original gamma ray. Various signal correction techniques are typically employed to improve the linearity and energy response uniformity in images in a manner to compensate for the inherent imperfections of a detector.
In gamma cameras, gamma-photon events registered directly from the radioactive source are discriminated from scattered events by an energy discriminating window enclosing the peak of energy of visible light originated by unscattered scintillation as detected by the photomultiplier tubes. When using multiple energy isotopes, several windows can be employed.
Camera sensitivity to the energy of an incident gamma photon will vary with position, i.e., the maximum energy (photopeak) in an energy spectrum of a large number of measured events will vary with position of the incident gamma photon. Furthermore, the width of the spectrum peak (i.e., energy resolution) will vary with position. Both of these factors influence the area of the spectrum that falls inside the energy windows and hence influence the sensitivity of the camera depending on position. The unequal sensitivity with position results in non-uniformities in the recorded image when exposed to a uniform gamma photo flux or to gamma emission in a medical study.
Partial energy correction has been obtained in the prior art by employing an energy modifier dependent on position to align the photopeak of an energy spectrum with the center of a constant window. Equivalently, the energy window could be corrected by an offset depending on position to achieve the same result. Although alignment of the photopeak of the energy spectrum with a constant window improves sensitivity to the desired events, the image still contains non-uniformities due the variations in the shape of the energy spectra (energy resolution) with spatial position.
To compensate for variations in sensitivity with position due to vary energy resolution, a position-dependent window width has been applied. However, this procedure is difficult to implement and usually requires additional hardware.
Accordingly, it is a principal object of the present invention to provide a gamma camera having substantial uniformity in sensitivity throughout an image.
it is a further object of the invention to achieve image uniformity using a fixed window size.
It is another object of the invention to provide a method for finding energy correction offsets which result in image uniformity.